This application will test the hypothesis that endogenous ligands for benzodiazepine receptors are involved in the physical and/or pathophysiologial regulation of plasma volume/tonocity. This hypothesis is based on four types of background information: (1) specific receptors for benzodiazepines exist, are readily quantitated and are uniquely localized in specific parts of the renal cortex and medulla; (2) the maximal number of these receptors (Bmax) is altered by two conditions that produce profound alterations in plasma volume/tonicity (diabetes insipidus and deoxycorticosterone-salt hypertension); (3) endogenous inhibitors of the benzodiazepine binding are present in ultrafiltrate of plasma, and we have purified from urine a putative endogenous ligand over 100,000 fold; (4) six lines of evidence suggest that benzodiazepine receptors are involved in regulation of plasma volume/tonicity. These six lines of evidence are: (a) Beta-carbolines are ligands for benzodiazepine receptors and produce vasopressin-like effects on sodium and water transport in amphibian skin; (b) benzodiazepines influence thirst; (c) diabetes insipidus alters renal benzodiazepine binding; (d) deoxycorticosterone-hypertension alters renal benzodiazepine binding; (e) the receptor is localized in nephron segments known to be important in the regulation of plasma volume/tonicity; and (f) two pharmaceutical companies have under study benzodiazepine derivatives that are diuretics. This hypothesis will be tested by: (1) determining the nephron segment(s) that contain the renal receptors for benzodiazepines; (2) purifying further the putative endogenous ligand we have found in urine in order to enable identification of the chemical nature of the compound; and (3) searching for effects of the putative endogenous ligand on the urinary excretion of Na, K+, Cl-, net acid and free water. The proposed studies hold high promise for yielding completely new information about regulation of renal function and the maintenance of plasma volume/tonicity, since there is no precedence for the existence of a functionless cellular receptor that undergoes regulation or for which there is an endogenous ligand.